Draglines are commonly used for removing large volumes of material, such as dirt, loosened ore, etc., and are particularly well-suited for removing overburden in large strip mining operations where tens of millions of yards of material must be removed in an efficient manner. A typical dragline is shown in FIG. 1. Draglines work by dragging a large bucket along the surface to scoop up material, hence the name. Draglines provide several advantageous features over other earthmoving equipment, including a long reach for both digging and dumping, the ability to dig below their tracks (or base), and a high cycle speed.
Draglines are available in a variety of different sizes, with the largest draglines being among the most massive mobile equipment ever produced. For example, the dragline shown in FIG. 1 is a Marion 8750 series dragline that has a 360 foot boom, and is equipped with a 135 cubic yard bucket. The largest dragline ever built has a bucket capacity of 220 cubic yards and weighs nearly 14,000 tons.
Referring to FIG. 1, the major components of a dragline include a powerplant 100, a boom 102, a hoist cable 104, a bucket 106, hoist chains 108, drag chains 110, dump cables 112, and drag cables 114. The machine powerplant 100 is mounted on a rotary base 115, allowing the boom to swing in the horizontal plane. Smaller draglines typically employ sets of tracks for moving the machine, while larger draglines use a "walking" mechanism. These larger machines are referred to as walking draglines. The hoist cable 104 can be retracted or extended by means of a hoist drum (not shown) that is located in the powerplant. Likewise, the drag cable 114 can be retracted and extended by means of a drag drum (not shown) located in the powerplant.
As shown in FIG. 2, the drag cable 114 is connected to pair of drag sockets 116. The drag sockets 116 are connected through drag devises 118 to the drag chains 110. The drag chains 110 are connected to the bucket 106 at hitch devises 120. The drag sockets 116 are also respectively connected to a pair of dump sockets 122 at dump devises 124. A second pair of dump sockets 126 is connected to the front of the bucket 106 at anchor links 128. The dump sockets 122 and 126 are commonly connected to a respective pair of dump cables 112 which ride on dump sheaves 130. A pair of upper hoist cables 132 are commonly connected to the bottom pickup link 134 at their top ends, and opposing sides of a spreader 136 at their bottom ends. A pair of lower hoist cables 138 are connected to the spreader 136 at their top ends, and are connected at their bottom ends to the bucket 106 at trunnions 139. The pickup link 134 is connected to a hoist equalizer 140, which in turn is connected to hoist sockets 142. The hoist sockets 142 are connected to the hoist cables 104. The hoist equalizer 140 is also connected to a pickup link 144, which is connected to a dump sheave shackle assembly 146 that holds the dump sheaves 130.
The loads on the hoist and drag chain links are massive. It is common for the largest draglines to employ hoist and drag cables that are 5 inches in diameter. These cables are made out of very high strength steels, and support suspended loads of upwards of 750,000 lbs. The loads placed on the hoist chains and drag chains are equally impressive. These loads dictate the use of specialized chain links made from ultra-high-strength alloyed steels. In addition, these chains and chain links must be designed to endure a tremendous amount of wear, as discussed below.
A typical dragline digging cycle is shown in FIGS. 3A-3F. As shown in FIG. 3A, the digging cycle begins by lowering the bucket into the mine pit with both the hoist cable and the drag cable nearly taut until the bucket contacts the pit surface. At this point the hoist cable is slightly slackened and the drag cable is pulled toward the powerplant (FIGS. 3B-3E). This results in the bucket teeth digging in and cutting a slice of material that piles inside the bucket. The depth and angle of the cut may be controlled by varying the hoist cable length as the drag cable is pulled.
The most important chain links in the hoist chains are the links that are in close proximity to the uppermost portion of the bucket sidewalls. It is common for these links to get damaged or worn when the spreader bar does not adequately prevent these links from hitting the sidewall of the bucket. Such a situation is shown by FIGS. 4A and 4B. In FIG. 4A the spreader 136 is shown in the ideal position, being centered above the bucket 106 so as to prevent contact between any of the chain links and the sidewalls 146 of the bucket. FIG. 4B shows the position of the spreader and hoist chains when the boom is swung before the bucket has been lifted clear of the surrounding material, a common occurrence during operation. In this case the chain link 147 adjacent the upper edge of the left-hand side of the side wall 146 contacts the left-hand upper sidewall 146 of the bucket at area 148. The links that so contact the bucket sidewall become so worn that they fail prior to the failure of the remainder of the links of the chain, and must be replaced, which is very costly in terms of material and downtime.
A similar contact between one of the chain links and the bucket sidewall can occur if the bucket does not track straight when it is being dragged, or if the bucket encounters a large boulder on one side, causing the bucket to rotate. As shown in FIGS. 3A-3F the chain link 150 moves back and forth adjacent to wear area 152. The chain link 150 wears against the wear area when the bucket is dragged while askew. To compensate for the wear, wear shoes (shrouds) are sometimes added to the upper sidewalls. However, this generally increases the contact between the chain links and bucket (at the shoes (in comparison to a bucket without shoes)), shortening the life of the chain links even further.
FIGS. 5A and 5B show a conventional scheme for compensating for the contact between the lower hoist chain links and the bucket sidewall. This scheme employs the use of two large barrel-shaped links 154, each of which provides a large surface area to wear against the bucket sidewalls. While these links provide an improved life over conventional links, they have the drawback of being significantly heavier than the links they replace. They also increase the bucket sidewall wear due to their larger diameter and barrel shape which results in instances of contact that would not occur with a conventional link.
In addition to the foregoing sidewall wear problems, conventional hoist chains are heavier than desired. This extra weight reduces the payload (the amount of material removed with each bucket load) the dragline can carry, and also increases the stress loading placed on the boom. The payload for a given machine is generally limited by the size of its bucket and the type of material the dragline is working in. The size of the bucket is limited by the maximum allowable suspended load rating of the machine, the suspended load including the weight of a loaded bucket and the weight of the various other components that are supported by the hoist cable (the hoist chains, drag chains, sockets, clevises, etc.--hereinafter the bucket support components). The suspended load rating is primarily a function of the strength of the boom, the torque capacity of the hoist drum and drag drum, and the overall horsepower of the machine.
The maximum suspended load rating for a machine is determined by performing an engineering analysis of the boom structure, using a safety factor that in part is determined by prior experience. It is generally desired to maximize the payload for a given machine, and this usually leads to using the machine at near its maximum suspended load rating. However, operating at near the maximum rating usually can only be performed on newer machines, because the strength of boom is reduced over the lifetime of the dragline. This is due to the constant fatigue loading that is applied to the boom during machine operation. The fatigue loading of the boom can be reduced by reducing the suspended load. Unfortunately, a reduction in the suspended load usually means a reduction in payload.
It would therefore be advantageous to be able to (1) maximize the payload without reducing the suspended load and/or (2) reduce the suspended load without reducing the payload capacity. The first object can be accomplished by increasing the size of the bucket in conjunction with a decrease the weight of the bucket support components. The second object can be accomplished by simply reducing the weight of the bucket support components while maintaining the bucket size.
Both of these objects can be achieved by reducing the weight of the hoist chains. Conventional hoist chain links are sized so that they will be able to support their loads after significant wear. The most common failure point of a hoist chain link is at its ends, which continuously wear on the ends of connecting links as the hoist chains are flexed during dragline operations. Thus, the dragline chain links are sized so that they will support their required load after significant wear of the end portions of the link. The nominal size of the chain links (generally a cross-section thickness) is primarily a function of the strength of the chain link material, the load that must be carried, and empirical wear data. As a result of the wear considerations, conventional hoist chain links are sized to be much larger (and heavier) than would be necessary to carry their nominal loads.
Although a reduction in the weight of the hoist chains is desired, such weight reduction has previously been limited because of the foregoing wear considerations. It is therefore desired to produce reduced-weight hoist chains that have similar performance (wear and strength) characteristics when compared with heavier conventional chains. Furthermore, it is also desired to have a chain link that is configured so as to lessen the contact with bucket sidewalls while maintaining or improving upon the life of the chain link (when compared to conventional hoist chain links), without requiring any additional weight.